The invention relates to a method for producing a multilayer coating for reflecting radiation in the soft X-ray or EUV wavelength range on an optical element and to an optical element which is operated at an operating temperature of 30° or more, preferably 100° C. or more, particularly preferably 150° C. or more, in particular of 250° C. or more, in an optical arrangement. The invention also relates to an optical arrangement comprising at least one optical element of this type.
Optical elements for the soft X-ray or EUV wavelength range (that is to say for wavelengths which typically lie between 5 nm and 20 nm) are used, inter alia, in optical arrangements such as projection exposure apparatuses for microlithography for the production of semiconductor components. Since no optical materials which have a sufficient transmission are known at these wavelengths, the optical elements are operated in reflection, the maximum reflectivity of the optical elements at these wavelengths typically being not more than approximately 70%, that is to say that approximately one third of the radiation impinging on a reflective optical element is not reflected and therefore does not impinge on the optical elements disposed downstream in the optical beam path. Accordingly, optical elements arranged further at the front in the beam path, that is to say closer to the EUV light source, are subjected to a considerably higher radiation loading and are heated to a much greater extent than is the case for optical elements situated further back in the beam path. Thus, by way of example, the temperature of a collector mirror which is the first optical element succeeding EUV light source, during operation of the optical arrangement, can amount to between approximately 200° C. and approximately 400° C. or higher. The temperature of the downstream optical elements in the illumination system can be 200° C. or more and temperatures of approximately 60° C. or higher can also occur even in the projection system of the optical arrangement.
The multilayer coatings of the optical elements used for reflecting radiation in the EUV wavelength range generally have alternate layers of materials having high and low refractive indices, e.g. alternate layers of molybdenum and silicon, the layer thicknesses of which are coordinated with one another in such a way that the coating can fulfill its optical function and, in particular, a high reflectivity is ensured. When the multilayer coatings of the optical elements are heated to the high temperatures mentioned above, however, thermal loadings of the multilayer coatings can occur which adversely affect the optical properties of the optical elements, as is explained in detail below.
WO 2007/090364 discloses that the substances molybdenum and silicon that are usually used as layer materials tend at high temperatures toward formation of molybdenum silicide as a result of interdiffusion processes at their interfaces, which leads to a reduction of the reflectivity on account of an irreversible decrease in the layer thickness of the layer pairs, which brings about a shift in the reflectivity maximum of the multilayer coating for the impinging radiation toward a shorter wavelength. In order to overcome this problem, WO 2007/090364 proposes using a silicon boride instead of silicon and a molybdenum nitride instead of molybdenum.
In order to solve this problem, DE 100 11 547 C2 proposes applying a barrier layer composed of Mo2C at the interfaces of the silicon and molybdenum layers, in order to prevent the interdiffusion between the layers and thereby to improve the thermal stability of the multilayer coating.
DE 10 2004 002 764 A1 in the name of the applicant discloses that the layers of a multilayer coating, when they are applied by means of specific coating methods, have an amorphous structure with a lower density than the corresponding materials as solids. The initially low density of the layers increases irreversibly at elevated temperatures, thus resulting in a reduction of the layer thicknesses of the individual layers and, in association with this, an increase in the period length of the coating. This likewise has the consequence that the wavelength at which the multilayer coating assumes a maximum of the reflectivity is shifted. In the extreme case, the period length can change to such an extent that the optical elements coated with the multilayer coating become unusable. In order to solve this problem, DE 10 2004 002 764 A1 proposes providing an oversize during the application of the layers, and anticipating the irreversible reduction of the layer thicknesses by heat treatment of the multilayer coating before the latter is used in an optical arrangement.